1. Field of the Invention
Exemplary aspects of the present invention generally relate to a transfer device and an image forming apparatus including the transfer device, and more particularly, to a transfer device including a toner scatter prevention mechanism and an image forming apparatus including the transfer device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; a transfer device transfers the toner image directly from the image bearing member onto a recording medium or indirectly from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In a known color image forming apparatus, a plurality of imaging forming stations, one for each of the colors, for example, cyan, magenta, yellow, and black, are disposed around a drum-type intermediate transfer member or disposed in tandem along a belt-type intermediate transfer member. Toner images formed in the image forming stations are primarily transferred onto the intermediate transfer member so that they are superimposed one atop the other, thereby forming a composite toner image on the surface of the intermediate transfer member. Subsequently, the composite toner image is transferred secondarily onto a recording medium, such as a sheet of paper.
When forming a color image, a toner density of the toner image needs to be adjusted to obtain a desired image quality. In the case of the belt-type intermediate transfer member (hereinafter referred to as an intermediate transfer belt), in order to achieve a desired toner density of developed images, a test image, also known as a pattern image, is formed on the intermediate transfer belt, and the toner density of the test image is detected by an optical detector, and the toner density of the image to be produced is then adjusted to the desired toner density by changing image forming conditions, which include charging properties, writing properties, a developing bias, and a density of the developing agent.
Generally, the test images are different from an actual image to be output on a recording medium. Thus, the test images are formed at specific times, such as between successive recording media sheets when recording media sheets are continuously fed, such as during continuous imaging operations. Furthermore, the test images are formed on the intermediate transfer belt in a manner similar to that of the actual image forming operation when the image forming operation for forming the actual image does not take place.
After the composite toner image is formed on the intermediate transfer belt in a primary transfer process, the composite toner image is transferred onto the recording medium in a secondary transfer process. In order to increase the efficiency of the secondary transfer, typically a transfer bias is supplied to a back tension roller disposed at the back of the intermediate transfer belt, opposite a secondary transfer member.
In this approach, a repulsive force acts on toner on the intermediate transfer belt to facilitate transfer of toner. More specifically, an electrostatic force from the back tension roller acts on the toner, thereby repelling the toner and hence facilitating transfer of the toner from the intermediate transfer belt to the recording medium.
However, there is a drawback to such a configuration in that the repulsive force acts also on the test images, causing the toner in the test images on the intermediate transfer belt to scatter directly toward the secondary transfer member. As a result, the scattered toner may contaminate the optical detector described above, disposed near the belt surface, that is, at the secondary transfer member side, resulting in detection failure. Furthermore, the scattered toner may stick to various components such as a sheet guide for guiding the recording medium from the secondary transfer portion and a charge eliminator for separating the recording medium from the intermediate transfer belt, resulting in contamination of the recording medium and poor separation of the recording medium from the intermediate transfer member.
To counteract this problem, a known approach to prevent contamination with the scattered toner due to the repulsive force employs a shield to cover the place where the test image is formed between successive recording media sheets being transported. The shield is then removed during image transfer.
In a case in which the test image is formed within an image forming area on the intermediate transfer belt, the shield is provided between the test image and the charge eliminator to prevent the test image formed on the intermediate transfer belt from being affected by electrical discharge from the charge eliminator. Upon transfer of the actual image, the shield is opened, allowing the electrical discharge from the charge eliminator to reach the recording medium so that the recording medium is separated from the intermediate transfer belt.
Although advantageous, a dedicated structure for opening and closing the shield is necessitated, increasing the size of the transfer device as a whole. Furthermore, a complicated control system for controlling the timing with which the shield is opened and closed is required, because the opening and closing of the shield needs to be synchronized with arrival of the test image or the actual image at the charge eliminator.
In another approach, an electrically conductive member is disposed opposite the intermediate transfer belt and connected to ground to collect the charged toner scattered from the intermediate transfer belt electrostatically. The conductive member serving as a dust collecting member, the conductive member is disposed downstream from a secondary transfer position to attract electrostatically the toner on the intermediate transfer belt.
In this configuration, however, because the detection position for the density of the test image is further back from the dust collecting position, some toner in the test image may be collected undesirably by the dust collecting member, affecting the accuracy of image density readings obtained by the optical detector.